This invention relates to the formation of polycrystalline alkali-metal-beta- and betaxe2x80x3- alumina ceramics, particularly for use as electrolytes in sodium-sulfur batteries, alkali metal thermoelectric converters (AMTEC) and alkali metal sensors.
One of the high-temperature secondary battery systems being investigated as a power source for electric vehicles is the sodium-sulfur battery. These batteries offer a high specific energy and high specific power, both of which are required for electric vehicles. These battery systems may also be potential energy storage devices for electric utilities, where long life and low cost are more important than high specific energy and high specific power.
In a sodium-sulfur cell, a liquid anode of metallic sodium and liquid cathode of sulfur or sodium polysulfide are separated by a polycrystalline ceramic electrolyte of either sodium beta- or betaxe2x80x3-Al2O3. The operating temperature is typically between 300 and 400xc2x0 C. In this battery, sodium ions diffuse during discharge from the anode to the cathode by ionic conduction through the ceramic electrolyte. Usually the electrolyte is in the form of a tube with the liquid sodium anode in the interior of the tube. For high operating efficiency and low battery cost, it is essential that the conductivity of the electrolyte be as high as possible. For this reason, the preferred electrolyte is the sodium betaxe2x80x3- Al2O3, because of its higher ionic conductivity.
The ceramic electrolyte is usually manufactured by mixing powders of Al2O3 and Na2O (along with either Li2O and/or MgO for stabilizing the betaxe2x80x3- phase) in appropriate proportions forming a powder compact and subjecting the powder compact to any one of various sintering processes. Examples of these processes are disclosed in xe2x80x9cSintering Processes and Heat Treatment Schedules for Conductive, Lithia-Stabilized Betaxe2x80x3-Al2O3xe2x80x9d by G. E. Youngblood, A. V. Virkar, W. R. Cannon, and R. S. Gordon: Bull Am. Ceram. Soc., 1977, 56. 206, and xe2x80x9cMaterials for Advanced High Temperature Secondary Batteriesxe2x80x9d by J. E. Battles: International Materials Reviews., 34. 1. In most sintering processes the formation of a grain boundary liquid phase is essential. The liquid phase formation can enhance the sintering kinetics, but results in a large grain size, which decreases the mechanical strength of the electrolyte. The liquid phase sintering also generates a residual NaAlO2 phase along the grain boundaries. The phase can react with moisture and further reduce the mechanical integrity of the ceramic electrolyte.
In U.S. Pat. No. 5,415,127 to Nicholson et al. and in xe2x80x9cFormation and characterization of Na-Betaxe2x80x3-alumina single crystal filmsxe2x80x9d by Aichun Tan, Chu Kun Kuo and Patrick S. Nicholson: Solid State Ionics 1993, 67. 131 is disclosed a method for the formation of Na-Betaxe2x80x3-Al2O3 single crystal films. These films, because of their optical properties, have potential in solid-state lasers, holography, signal and image processing, phosphor chemistry, and other optical devices. The process comprises providing a single crystal substrate of alpha-alumina with an optically smooth surface parallel to a (001) crystal plane, and heating the substrate in the presence of a vapor containing Na2O to react with the alumina and a stabilizing ion, such as lithium. The polished surface is required to form a single crystal. Otherwise a polycrystalline material is formed. The alumina is converted to Na-betaxe2x80x3-Al2O3 as it reacts with sodium oxide from the vapor. After conversion to Na-betaxe2x80x3-Al2O3 on the surface, further conversion requires that the Na2O in the form of ions be transported through the already formed Na-betaxe2x80x3-Al2O3 and react with the alumina at a reaction interface. This provides a slow moving reaction front moving through the substrate. The kinetics of this process is rather sluggish. Disclosed is conversion of a 40 xcexcm thickness at 1600xc2x0 C. in one hour. To convert a 0.5 mm thick electrolyte plate or tube for a battery electrolyte, several hundred hours at xe2x89xa71450xc2x0 C. would be required. This long time of formation materially adds processing and equipment costs to the fabrication. The limiting step is believed to be the diffusion of oxygen ions through the converted Na-betaxe2x80x3-alumina material, since the only species that exhibits high mobility in Na-betaxe2x80x3-alumina is the sodium ion. To convert alumina into Na-betaxe2x80x3-alumina, both sodium and oxygen are required, and the diffusion of oxygen through Na-betaxe2x80x3-alumina is very slow.
Materials analogous to Na-beta-alumina and Na-betaxe2x80x3-alumina also have been found to be useful in various processes. For example, in xe2x80x9cPotassium Betaxe2x80x3-Alumina Membranesxe2x80x9d G. M. Grosbie and G. J. Tennenhouse: Journal of the American Ceramic Society, 65, 187 is disclosed membranes of the potassium analogs, K-Beta-alumina and K-Betaxe2x80x3-alumina, that are made by ion-exchanging the sodium materials. These potassium materials have potassium-ion conductivity and may be used where the potassium-ion conductivity is required.
Sodium and potassium-beta-alumina and betaxe2x80x3-alumina materials are also used in applications other than for battery applications, such as for sodium heat engines (SHE) or in general alkali-metal thermoelectric converters (AMTEC). Because of the continuing interest in these and the above battery technologies, a method for quickly producing electrolytes and other shapes without the disadvantages of sintered shapes would be an advance in the art.
It is, therefore, an object of the invention to provide a method for forming alkali-metal-beta- and betaxe2x80x3-alumina and gallate materials that do not require sintering and that avoids the formation of increased grain size and of grain boundary liquid phase.
Another object of the invention is to provide a method for forming alkali-metal-beta- and betaxe2x80x3-alumina and gallate materials by diffusion wherein oxygen diffusion is accelerated and is not the limiting step.
Another object of the invention is to provide a method shortening the processing time in the formation alkali-metal-beta- and betaxe2x80x3-alumina and gallate products by diffusion.
Further objects of the invention will become evident in the description below.
The present invention is a composition and method for forming for alkali-metal-beta- and betaxe2x80x3-alumina and gallate products. The method of the invention allows conversion of alpha-alumina into an alkali-metal-beta- or betaxe2x80x3-alumina at a rate which is ten to hundred times larger than diffusion methods as disclosed in the Nicholson et al. patent.
The invention also involves the production of alkali-metal-beta- or betaxe2x80x3-gallate from gallium oxide, Ga2O3. Gallium has an analogous chemistry to aluminum as it applies to the present invention and composites of the invention can be made from alpha-Al2O3 or Ga2O3, or mixtures thereof. Accordingly, it is understood that any disclosure of the present invention for the production of alkali-metal-beta- or betaxe2x80x3-alumina from alpha-alumina also applies to the production of alkali-metal-beta- or betaxe2x80x3-gallate from gallium oxide. Mixtures of alpha-Al2O3 and Ga2O3 in the initial precursor composite, in which case a mixture of alkali-metal-beta- or betaxe2x80x3-Al2O3 and alkali-metal-beta- or betaxe2x80x3-Ga2O3 is formed, usually as a solution of the two.
The method of the invention comprises making a composite of alpha-alumina and an oxygen-ion conductor, such as zirconia, and then exposing it to a vapor containing an alkali-metal oxide, preferably an oxide of potassium or sodium, more preferably an oxide of sodium. The alkali-metal-beta- or betaxe2x80x3-alumina may be of any of the alkali metals, including rubidium (Rb), cesium (Cs), and lithium (Li), but preferably sodium (Na) and potassium (K), more preferably sodium (Na).
The vapor may also contain one or more stabilizers to inhibit transformation of betaxe2x80x3-alumina to the beta-alumina. These include, but are not limited to MgO, Li2O and ZnO. Other suitable stabilizer for betaxe2x80x3-alumina or beta-alumina may also be added. Alternately, one or more stabilizers may be included in the formation of the alpha-alumina/oxygen-ion conductor composite.
The oxygen-ion conductor may be any suitable ceramic oxygen-ion-conducting material. Examples include, but are not limited to, known oxygen-ion conductors, such as zirconia and its various forms, e.g., yttria stabilized zirconia, rare-earth-oxide-doped zirconia, and scandia-doped zirconia, and ceria ceramics, e.g., rare-earth oxide doped ceria and alkaline-earth oxide doped ceria, stabilized hafnia, and thoria.
The alpha-alumina and oxygen-ion conductor are formed into a ceramic composite by any conventional method for green forming, such as, for example, pressing, extrusion, slip casting, injection molding, tape casting, and the like, followed by sintering or hot-pressing. The physical properties of the final product derive in large part from those of the initial ceramic composite. Accordingly, fabrication methods that produce high-strength, fine-grained materials are preferred.
Both, alpha-alumina and the oxygen-ion conductor are present in amounts to form continuous matrices of alpha-alumina phase and the oxygen-ion conductor phase. This provides two continuous, penetrating networks. Ceramic shapes of 30 to 70 vol. % alpha-alumina and 70 to 30 vol. % oxygen-ion conductor have been generally found suitable. Shapes of a composition outside of these limits may also be suitable if a continuous matrix of both the alpha-alumina and oxygen conductor constituents is present.
The alpha-alumina/oxygen conductor ceramic shape is exposed to the appropriate ion species in the form of an alkali-metal-oxide at an elevated temperature, above about 800xc2x0 C., preferably between 1200xc2x0 C. and 1500xc2x0 C. If the temperature is too low, the rate of reaction is inadequate. If the temperature is too high there can be an evaporation loss of the alkali-metal oxide. Additionally, it is also possible for alkali-metal-betaxe2x80x3-alumina to convert into alkali metal-beta-alumina, especially above about 1600xc2x0 C.
The vapor contains an oxide of the alkali-metal, preferably as Na2O, if the sodium form is desired, and potassium, preferably as K2O, if the potassium form is required. The vapor also contains a stabilizing ion, preferably as Li2O, MgO, ZnO if the betaxe2x80x3- form is desired. The process of the invention is preferably carried out by embedding the composite shape of alpha-alumina and oxygen-ion conductor in a powder that when heated to reaction temperature produces the appropriate vapor. For example, a powder of alumina and alkali-metal-beta or - betaxe2x80x3-alumina, or a powder of alumina and alkali-metal oxide calcined to form alkali-metal-beta or -betaxe2x80x3- alumina is suitable. Any other suitable process for exposing the composite to alkali-metal vapors is contemplated.
During the process of the invention, oxygen ions transport through the oxygen-ion conductor while sodium ions transport through the already formed alkali-metal-beta- or betaxe2x80x3-alumina. In this manner, rapid paths are provided for both species and the reaction kinetics are not controlled by the rates of diffusion, but primarily by the formation of alkali-metal-beta- or betaxe2x80x3-alumina at the reaction front; that is the boundary separating alpha-alumina (with oxygen-ion conductor) and formed alkali-metal-beta- or betaxe2x80x3-alumina (with oxygen-ion conductor).
The final product is a ceramic composite of the oxygen-ion conductor and the beta-or betaxe2x80x3-alumina. Since no sintering conditions were required to form the beta- or betaxe2x80x3-alumina, there is no formation of liquid phases that compromise the properties of the composite. In addition, the oxygen-ion conductor can be chosen to contribute positively to the physical properties, providing a further enhancement.